A rectenna (rectifying antenna) is a special type of antenna that is used to convert radio wave energy into direct current electricity. Rectennas are used in wireless power transmission systems that transmit power by radio waves. A typical rectenna element consists of a dipole antenna with a diode connected across the dipole elements. The diode rectifies the alternating current (AC) induced in the antenna by the radio waves to produce direct current (DC) power, which is then used to power a load connected across the diode. Schottky diodes are usually used because they have the lowest voltage drop and highest speed and therefore have the lowest power losses due to conduction and switching. Large rectennas consist of an array of many such dipole elements.
There exists a significant motivation to develop rectennas that efficiently capture RF energy. Due to the increasing use of radio frequency (RF) signals to transmit information, the presence of RF energy is almost ubiquitous. Moreover, the unused portions of transmitted RF signals (i.e., portions that are not captured and converted by a receiver) are essentially “wasted” in that the energy transmitted in unused RF signals is absorbed in ambient matter and thus lost. As such, in addition to providing power sources for remote/portable devices, a suitable rectenna capable of efficiently re-converting unused ambient RF energy would reduce overall energy demand by replacing batteries and other remote power sources.
Conventional rectennas capable of converting RF energy to DC power are unable to harvest most ambient RF energy. Conventional rectifying circuits contain a distinct RF matching stage for matching the impedance of the antenna (typically a standard 50 or 75 Ohm antenna) to the input impedance of the rectifying circuit (typically with a strong inductance/reactance in addition to the resistance). Accordingly, existing RF rectennas can only produce usable DC voltages in the presence of high RF power density levels (typically above 1 W/m2), and are therefore typically used in wireless power transmission systems where dedicated high-power transmitters generate RF energy at sufficiently high energies. Ambient RF energy is typically very low (from mW down to μW), and thus the use of conventional RF rectennas for energy harvesting has proven to be insufficient for low-power levels ambient sources. Also, conventional rectennas are single band (i.e., harvest narrow-band RF signals) and are bulky, and therefore not scalable.
What is needed is an RF energy harvesting device (rectenna) that is capable of harvesting ambient RF energy. In particular, what is needed is a rectifying circuit that allows an RF rectenna to harvest multiband RF signal energy having low energy levels (i.e., tens of mW and below), and with high conversion efficiencies.